FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) Title of invention: PREPARATION METHOD OF SEPARATOR, SEPARATOR FORMED THEREFROM, AND ELECTROCHEMICAL DEVICE CONTAINING THE SAME Applicants: LG CHEM, LTD. A company incorporated in Republic of Korea having address: 128, Yeoui-daero Yeongdeungpo-gu Seoul 150-721, Republic of Korea and TORAY BATTERY SEPARATOR FILM CO., LTD. A company incorporated in Japan having address: 1190-13, Iguchi, Nasushiobara-shi Tochigi 329-2763, Japan The following specification particularly describes the invention and the manner in which it is to be performed. 2 Technical Field [001] The present disclosure relates to a method of preparing a separator of an electrochemical device such as a lithium secondary battery, a separator formed therefrom, and an electrochemical device comprising the same. [002] The present application claims priority to Korean Patent Application No. 10- 2014-0038729 filed in the Republic of Korea on April 1, 2014, the disclosures of which are incorporated herein by reference. Background Art [003] Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, laptop computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention, and among them, the development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development for new electrode and battery design is being conducted to improve the capacity density and specific energy of the batteries. [004] Among currently available secondary batteries, lithium secondary batteries developed in the early 1990’s have received a great deal of attention due to their advantages of higher operating voltages and much higher energy densities than traditional batteries using aqueous electrolyte solutions, such as Ni-MH batteries, Ni-Cd batteries and H2SO4-Pb batteries. However, such lithium ion batteries have disadvantages of safety-related problems caused by the use of organic electrolyte solutions, for example, ignition and explosion, and complex manufacturing. Lithium ion polymer secondary batteries designed to overcome the weak points of lithium ion batteries are stated to be one of the next-generation batteries, but their capacity is still lower than that of lithium ion batteries and a discharge capacity, particularly, at low temperature, is insufficient, and accordingly, there is an urgent demand for improvement. [005] Such electrochemical devices are produced by many companies, but their safety characteristics show different aspects from each other. Assessing and ensuring the safety of electrochemical devices is very important. One of the most important considerations is that electrochemical devices should not cause damage to users in the event of malfunction, and for this purpose, Safety Standards impose strict regulations on ignition and explosion in 3 electrochemical devices. In the safety characteristics of electrochemical devices, electrochemical devices have a high risk of explosion in the event of overheat or thermal runaway of an electrochemical device or penetration of a separator. Particularly, a polyolefin-based porous polymer substrate commonly used as a separator of an electrochemical device shows serious thermal contraction behaviors at the temperature higher than or equal to 100oC due to material characteristics and procedural characteristics including stretching, causing a short circuit between a positive electrode (or cathode) and a negative electrode (or anode). [006] To solve the safety problem of electrochemical devices, a separator with a porous organic-inorganic coating layer formed by coating a mixture of excess inorganic particles and a binder polymer on at least one surface of a porous polymer substrate having plural pores was proposed. The inorganic particles included in the porous organic-inorganic coating layer have good heat resistance, thereby preventing a short circuit between a positive electrode (or cathode) and a negative electrode (or anode) when an electrochemical device is overheated. [007] Generally, a separator with a porous organic-inorganic coating layer is manufactured through a process which forms an organic-inorganic coating layer on a porous polymer substrate by dip coating. However, due to the use of an organic solvent-based slurry, this manufacturing method has safety hazards in the manufacture of an electrochemical device, and is less environmentally friendly and economically efficient. [008] As opposed to an organic solvent-based slurry, an aqueous slurry is safe, eco-friendly, and economically efficient, but its high surface tension causes a low wettability problem on a polyolefin-based substrate, limiting the use for separator coating. DISCLOSURE Technical Problem [009] Therefore, an object of the present disclosure is to provide a preparation method of a separator, by which a separator with an organic-inorganic composite porous coating layer is made through a process of coating a slurry on a porous polymer substrate, and in this instance, an aqueous slurry having predetermined properties is used to improve wetting properties on the porous polymer substrate. [010] Another object of the present disclosure is to provide a separator obtained by the preparation method. [011] Still another object of the present disclosure is to provide an electrochemical 4 device with the separator. Technical Solution [012] To achieve the above objects, the present disclosure provides a preparation method of a separator including preparing an aqueous slurry including inorganic particles, a binder polymer, and an aqueous medium, and coating the aqueous slurry on at least one surface of a porous polymer substrate to form an organic-inorganic composite porous coating layer, wherein capillary number of the aqueous slurry is from 0.3 to 65, the capillary number is determined by the following equation 1: Capillary number (Ca) = (μ × U) / σ where μ = viscosity (kgf•s/m2), U = coating velocity (m/s), and σ = surface tension (kgf/m). [013] According to an embodiment, there is provided a separator obtained by the preparation method. [014] According to an embodiment, there is provided an electrochemical device including a positive electrode (or cathode), a negative electrode (or anode), and the separator interposed between the positive electrode (or cathode) and the negative electrode (or anode). [015] According to an exemplary implementation, the electrochemical device may be a lithium secondary battery. Advantageous Effects [016] The present disclosure may ensure safety and economical efficiency in the process by providing a preparation method of a separator which forms an organic-inorganic composite coating layer on a surface of a porous polymer substrate using an aqueous slurry, and may improve the safety of an electrochemical device with the separator obtained as above. MODE FOR CARRYING OUT THE INVENTION [017] Hereinafter, the present disclosure will be described in detail. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted 5 based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. [018] A preparation method of a separator according to one embodiment includes preparing an aqueous slurry including inorganic particles, a binder polymer, and an aqueous medium, and coating the aqueous slurry on at least one surface of a porous polymer substrate to form an organic-inorganic composite porous coating layer. [019] The capillary number of the aqueous slurry may be determined by the following equation 1: Capillary number (Ca) = (μ × U) / σ where μ denotes a viscosity (kgf•s/m2), U denotes a coating velocity (m/s), and σ denotes a surface tension (kgf/m). [020] The capillary number is a factor for determining wettability of the aqueous slurry, and by suitably controlling this, wetting of the aqueous slurry on the porous polymer substrate may be achieved and coating may be facilitated. [021] The capillary number is determined by the viscosity, the coating velocity and the surface tension of the aqueous slurry, and the viscosity may change based on the solids content and the supply temperature of the aqueous slurry. The coating velocity represents a velocity when coating the aqueous slurry on the substrate, and based on the coating velocity, wetting of the aqueous slurry on the substrate may change. When the surface tension has a low value, wetting is achieved, making it more advantageous for applying the slurry, and this value may be minimized through an additive. [022] To ensure optimum wettability of the aqueous slurry through the capillary number, the value determined by the above equation 1 may have a range between 0.3 and 65, for example, between 0.5 and 45. Within this range, wetting on the porous polymer substrate with a low surface tension may be achieved and coating may be facilitated. [023] The viscosity of the aqueous slurry may be adjusted to an optimum range, and in this instance, a thickening agent may be used. The thickening agent is not limited to a particular type if it is a material able to adjust the viscosity of the aqueous slurry, but may include, for example, carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinylalcohol, oxidized starch, phosphorylated starch, casein, and their salts. These exemplary thickening agents be used singularly or in combination at any ratio. When the thickening agent is used, a ratio of the thickening agent to the aqueous slurry is 6 generally 0.1 mass% or more relative to the inorganic matter, preferably 0.5 mass% or more, more preferably 0.6 mass% or more, and an upper limit is generally 5 mass% or less, preferably 3 mass% or less, more preferably 2 mass% or less. When the ratio is less than the range, coating often reduces noticeably. When the ratio is more than the range, the content of the inorganic particles or binder in the slurry is likely to reduce. [024] The viscosity of the aqueous slurry may change based on the supply temperature, and the viscosity increases with the decreasing temperature and decreases with the increasing temperature. Thus, by selecting an optimum supply temperature, the viscosity of the aqueous slurry may be suitably controlled. The supply temperature range may be, for example, from 10oC to 50oC. [025] The viscosity of the aqueous slurry adjusted through the thickening agent may have a range between 0.005 kgf•s/m2 and 0.05 kgf•s/m2, for example, between 0.01 kgf•s/m2 and 0.25 kgf•s/m2. [026] The coating velocity of the aqueous slurry may be suitably adjusted to a range within which coating of the aqueous slurry on the porous polymer substrate is easily made, that is, a range within which wettability is ensured. The coating velocity may be determined by the mechanical control in the process, for example, a variable such as the velocity or tension of rollers, and may have a range between 10 m/s and 100 m/s, for example, between 30 m/s and 70 m/s. When the coating velocity is out of the range, economical efficiency reduces due to degradation in coating of the aqueous slurry or a prolonged process time. [027] The surface tension of the aqueous slurry is an important factor for ensuring wettability on the porous polymer substrate with low surface energy, and it is preferred to impart a low surface tension as possible. To do so, introduction of an additive into the aqueous slurry may induce a reduction in surface tension, and the additive may include an emulsifier, to be exact, a hydrophilic surfactant, for example, any least one hydrophilic surfactant selected from the group consisting of polyoxyethylene(10)-hydrogenated castor oil, polyoxyethylene(40)-hydrogenated castor oil, polyoxyethylene(60)-hydrogenated castor oil, siloxane, polysorbate 60, polysorbate 80, and polysorbate 20. The additive may be added at a content of from about 0.1 wt% to about 3.0 wt% for the weight of the aqueous slurry. [028] The surface tension of the aqueous slurry through the additive may be adjusted to a range between 0.0015 kgf/m and 0.007 kgf/m. [029] The aqueous medium used in the aqueous slurry may be water, or a mixture of alcohol and water. The alcohol may include, but is not limited to, methanol, ethanol, propanol, isopropanol, butanol, t-butanol, and pentanol. 7 [030] The porous polymer substrate being coated using the aqueous slurry may be a porous polymer film substrate or a porous polymer non-woven substrate. [031] As the porous polymer film substrate, as well known, a separator made of a porous polymer film of polyolefin such as polyethylene and polypropylene may be used, and the polyolefin porous polymer film substrate exerts a shut-down function at the temperature, for example, between 80oC and 130oC. The polyolefin porous polymer film may be made from polyolefin-based polymers including polyethylene such as high density polyethylene, linear low density polyethylene, low density polyethylene, and ultra high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, singularly or in combination. Also, the porous polymer film substrate may be manufactured using various polymers including polyester, as well as polyolefin. Also, the porous polymer film substrate may be formed in a stack structure of at least two film layers, and each film layer may be formed from polymers, for example, polyolefin and polyester as described above, singularly or in combination. [032] The porous polymer non-woven substrate may be manufactured from fibers using polymers including polyolefin-based polymers as described above or other polymers with higher heat resistance, for example, polyester such as polyethyleneterephthalate (PET). Similarly, the porous polymer non-woven substrate may be manufactured from the fibers, singularly or in combination. [033] The material or shape of the porous polymer film substrate may be variously selected based on the desired purpose. [034] There is no particular limitation on the thickness of the porous polymer substrate, but a preferred thickness is in a range of 1 μm to 100 μm, more preferably, 5 μm to 50 μm, and there is no particular limitation on the pore size and porosity of the porous polymer substrate, but the pore size and porosity is preferably from 0.01 μm to 50 μm and from 10% to 95%, respectively. [35] There is no particular limitation on a method of coating the aqueous slurry on the porous polymer substrate, but it is preferred to use a slot coating method or a dip coating method. The slot coating is a coating method which coats a coating solution supplied through a slot die on a front surface of a substrate, and a thickness of a porous coating layer may be controlled based on a flow rate being supplied from a quantitative pump. The dip coating is a coating method which immerses a substrate in a tank containing a coating solution, and a thickness of a porous coating layer may be controlled based on a concentration of the coating solution and a velocity when taking the substrate from the coating solution tank, and for more 8 accurate coating thickness control, post-metering through a mayer bar after immersion may be performed, and subsequently, drying in an oven is performed to form a porous coating layer on both surfaces of a porous polymer substrate. [036] In the slurry in which the inorganic particles are dispersed and the binder polymer is dissolved or dispersed in the aqueous solvent, the inorganic particles are not particularly limited if they are electrochemically stable. That is, available inorganic particles of the present disclosure are not limited to a particular type if they do not cause oxidation and/or reduction reactions in an operating voltage range (for example, from 0 to 5V for Li/Li+) of an electrochemical device being applied. In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, such inorganic particles may contribute to the increase in the degree of dissociation of an electrolyte salt, for example, a lithium salt, in a liquid electrolyte and may improve ionic conductivity of an electrolyte solution. [037] By the above reasons, preferably the inorganic particles include inorganic particles having a high dielectric constant greater than or equal to 5, preferably, greater than or equal to 10. The inorganic particles having a dielectric constant greater than or equal to 5 include, as a non-limiting example, BaTiO3, Pb(Zr,Ti)O3(PZT), Pb1-xLaxZr1-yTiyO3(PLZT, 0